1. Field of the Invention
The present invention relates, generally, to light-emitting devices and, more particularly, to light-emitting and detecting devices. Specifically, the present invention relates to improved monolithic devices which are capable of emitting and detecting light over a wide wavelength range. These devices are referred to as multi-color optical transceivers.
2. Description of the Prior Art
Optically coupled circuits and optical communication systems require high-efficiency light emitters and detectors which operate within a wavelength range which is suitable for the particular application. For fiber-optic communication systems utilizing silica fibers, the optimum wavelength range of interest is 1.0-1.5 .mu.m (i.e., an energy range of 0.83-1.24 eV). In current systems the data transmission rate is limited by several factors including the inability of emission/detection devices to transmit/receive at more than a single wavelength. The monochromatic nature of present optical transceivers also results in complex optical interconnects because separate fibers are required for simultaneous transmission and reception.
Photovoltaic cells are known which essentially comprise semiconductors that have the capability of converting electromagnetic energy (such as light or solar radiation) directly to electricity. Such semiconductors are usually characterized by solid crystalline structures that have energy band gaps between their valence electron bands and their conduction electron bands. When light is absorbed by the material, electrons that occupy low-energy states become excited to cross the band gap to higher energy states. For example, when electrons in the valence band of a semiconductor absorb sufficient energy from photons of the solar radiation, they can jump the band gap to the higher energy conduction band.
Electrons which are excited to higher energy states leave behind them unoccupied low-energy positions or holes. Such holes can shift from atom to atom in the crystal lattice and thereby act as charge carriers, as do free electrons in the conduction band, and contribute to the crystal's conductivity. Most of the photons absorbed in the semiconductor give rise to such electron-hole pairs which generate the photocurrent and, in turn, the photovoltage exhibited by the cell.
As is known, the semiconductor is doped with a dissimilar material to form p-type and n-type materials, which can be then used to form p/n junctions. A voltage across the junction, which is the photovoltage, is produced when the semiconductor is exposed to light energy.
It is known that photon energies in excess of the threshold energy gap or band gap between the valence and conduction bands are usually dissipated as heat; thus they are wasted and do no useful work. More specifically, there is a fixed quantum of potential energy difference across the band gap in the semiconductor. For an electron in the lower energy valence band to be excited to jump the band gap to the higher energy conduction band, it has to absorb a sufficient quantum of energy, usually from an absorbed photon, with a value at least equal to the potential energy difference across the band gap.
The semiconductor is transparent to radiation which has photon energies less than the band gap. On the other hand, if the electron absorbs more than the threshold quantum of energy, e.g., from a higher energy photon, it can jump the band gap. The excess of such absorbed energy over the threshold quantum required for the electron to jump the band gap results in an electron that is higher in energy than most of the other electrons in the conduction band. The excess energy is eventually lost in the form of heat. The net result is that the effective photovoltage of a single band gap semiconductor is limited by the band gap.
Thus, in a single semiconductor solar cell, to capture as many photons as possible from the spectrum of solar radiation, the semiconductor must have a small band gap so that even photons having lower energies can excite electrons to jump the band gap. This, of course, involves attendant limitations. First, use of a small band gap material results in a low photovoltage for the device and, naturally, lower power output occurs. Second, the photons from higher energy radiation produce excess energy which is lost as heat.
On the other hand, if the semiconductor is designed with a larger band gap to increase the photovoltage and reduce energy loss caused by thermalization of hot carriers, then the photons with lower energies will not be absorbed. Consequently, in designing conventional single-junction solar cells, it is necessary to balance these considerations and try to design a semiconductor with an optimum band gap, realizing that in the balance there has to be a significant loss of energy from both large and small energy photons.
Much work has been done in recent years to solve this problem by fabricating tandem or multijunction (cascade) solar cell structures in which a top cell has a larger band gap and absorbs the higher energy photons, while the lower energy photons pass through the top cell into lower or bottom cells that have smaller band gaps to absorb lower energy radiation.
The band gaps are ordered from highest to lowest, top to bottom, to achieve an optical cascading effect. In principle, an arbitrary number of subcells can be stacked in such a manner; however, the practical limit is usually considered to be two or three. Multijunction solar cells are capable of achieving higher conversion efficiencies because each subcell converts solar energy to electrical energy over a small photon wavelength band over which it converts energy efficiently.
Various electrical connectivity options between subcells are possible, including (1) series connected, (2) voltage matched, and (3) independently connected. In the series connected type of tandem solar cells, there is current matching of the two subcells. The advantage of the independently connected type is that it avoids the problems of having to electrically connect the two subcells. This type also allows more possibilities in designing the solar cell. However, it is more complex with respect to fabrication of the solar cell, and it is also more complex in terms of delivering the power from each separate cell to a single electrical load. This is a systems problem.
Such tandem cells can be fabricated in two different manners. The first manner involves separately manufacturing each solar cell (with different band gaps) and then stacking the cells mechanically in optical series by any of a number of known methods. The disadvantage of this method is due to the complexity in forming such a stacked arrangement. The advantage is the flexibility of being able to stack different materials on top of each other.
The second manner of fabricating a tandem solar cell involves forming a monolithic crystalline stack of materials with the desired band gaps. The advantage of this method is the simplicity in processing. The disadvantage is that there are a limited number of materials combinations which can be epitaxially grown in device-quality form.
It has been generally accepted by persons skilled in the art that the desired configuration for monolithic multijunction tandem devices is best achieved by lattice matching the top cell material to the bottom cell material. Mismatches in the lattice constants create defects or dislocations in the crystal lattice where recombination centers can occur to cause the loss of photogenerated minority carriers, thus significantly degrading the photovoltaic quality of the device. More specifically, such effects will decrease the open-circuit voltage (V.sub.oc), short circuit current (J.sub.sc), and fill factor (FF). Thus, the lattice-matched monolithic approach provides an elegant manner for the construction of a high-quality tandem cell.
U.S. Pat. No. 4,829,345 describes an electronic device which includes a light transmission system for transferring signals between the circuit parts by the use of light. The device requires use of a superlattice structure to achieve multiple wavelength emission and detection of light.
It is known that similarities exist between the design and operational characteristics of solar cells, photodetectors and light-emitting diodes. However, there has not heretofore been provided a monolithic multi-color transceiver having the advantages and desirable combination of features which are exhibited by the devices of the present invention.